Many methods exist for controlling operation of a combustion engine. In particular, measurements of pressure from one or more cylinders of an engine are commonly used as a basis for engine performance measurement, monitoring and for control of an engine.
One area of use for continually measuring pressure sensors is in very large combustion engines which are operated at relatively low revolutions per minute. Such motors are used for example as ships engines and as stationary engines for driving electrical generators and gas compressors. However engine monitoring and control is also increasingly applied to smaller engines of the type such as those used in ordinary vehicles such as cars, trucks and buses.
Because of the growing demands for reduced consumption of fuel and continually increasing environmental demands on the chemical composition of exhaust gases the requirement to monitor the operation of combustion engines has increased. Misfiring influences exhaust gas chemical composition and can also negatively influence the working life of a combustion engine. With the help of continuous measurement misfiring and other factors affecting engine performance can be detected and action be taken to ensure proper functioning is regained.
A difficulty in the calculation of work output, IMEP of a cylinder is to determine accurately a crank angle for a piston together with the pressure corresponding to that crank angle. Generally crank angle of a given piston is calculated from a measurement of the angular position of the crankshaft of the engine (crankshaft angle), as it is difficult to measure the piston position directly in the cylinder during operation. The crankshaft angle is usually obtained by an independent measurement of the angular position of the flywheel (flywheel angle) relative to a known position such as a timing mark, according to known methods.
Top dead centre (TDC) of a piston in a cylinder may be determined mechanically according to known methods by measuring with, for example, a capacitive sensor or a position transducer in an engine at rest. Another method includes shutting off fuel to a given cylinder of a motor during operation, measuring the cylinder pressure, and estimating the position of TDC in that cylinder from an expected symmetry of the measured pressure curve. In this method a maximum of the symmetrical pressure curve indicates TDC, although that indicated maximum point may contain thermodynamical errors. A principal disadvantage with both of these methods is that they do not provide for determination of TDC in a motor under normal operation.
A method and apparatus described in JP 9329049 discloses that a value for IMEP may be calculated dependent on measured cylinder pressure and known crankshaft angle, and that such a value may be used for control or regulation of fuel supply amount and ignition timing for a motor vehicle engine.
What is difficult in practice with most methods described is to determine accurately an angular offset that usually exists between the flywheel angle and the piston crank angle at a given moment in a revolution. In order to determine the offset it is usually considered sufficient to determine the flywheel angle in relation to a known position of the piston in the cylinder, and the position commonly used is the top dead centre position in the cylinder. The TDC position corresponds to a crank angle for a piston of 0 degrees.
However, the usual methods rely on a measurement based on a point marked at the periphery of the flywheel, which measurement typically depends on a position of a train of mechanically connected engine components in an engine at rest.
For instance in EP 742 359 A2, a method and apparatus for controlling the operation of an internal combustion engine is described. In this description the crankshaft angle is measured by a sensor detecting gear teeth on a ring gear fixed to the flywheel of an engine.
However manufacturing tolerances of engine parts mean in practice that dimensions of parts vary somewhat from specified or expected dimensions. In addition in an engine under operation the dynamic interplay of stresses on components, free play clearances between mechanically connected components, friction and inertia of components means that the mechanical distances and relationships between components under operation may not be the same as distances and relationships between the same components at rest. The relative position of mechanically connected piston and a flywheel at rest does not provide a measurement of the relative position of the same piston and flywheel, as a basis for a computation of the crank angle during operation of an engine, to a degree which is sufficiently accurate for modern engine diagnostic systems or engine control systems.